There's so much good information above, I can hardly add to the facts already presented. However, a bit of personal experience may help.
I've always used the "traditional" Pentax cameras from K200D to K30 and on through K5 & K3. When the KP came out, I couldn't decide if I liked or disliked the different design. I was so much concerned that, not being able to put my hands on one before buying it, I was so hesitant to buy one that I put it off, even through the nice sale type offers when it was being closed out.
Well, fast-forward to now: I recently had the chance to buy a used KP at a good price. I've simply been amazed (maybe I shouldn't have been?) at how easily I adapted to it. For whatever reasons, the KP feels "just right" in my hands (with the large grip). The controls are a natural (for me) to figure out. It's "very Pentax" despite looking a bit different. I even find the shutter release to be a natural position, maybe tied to film photography days.
My conclusion is that, personally, I'd prefer a lightly used KP to a new K70.
There were a couple on Adorama a few days ago. I see someone has bought the more expensive one. The remaining is an E- condition, with box, battery, and charger for $589.
https://www.adorama.com/us1506892.html?